The present invention relates to double conversion television tuners, and more particularly, to such tuners that have split IF (intermediate frequency) outputs.
The conventional television receiver is of the "intercarrier" type, wherein both the amplitude modulated (AM) picture and frequency modulated (FM) sound carriers are amplified in a common IF amplifier. A video detector provides an output signal comprising a composite video signal and an intercarrier sound signal. The intercarrier sound signal is then applied to an FM detector to provide a baseband sound signal. In the intercarrier system, due to the mixing of the AM picture carrier and the FM sound carrier to obtain the intercarrier sound signal, the amplitude modulation of the picture signal may cause a buzz at the horizontal frequency and harmonics thereof. However, for monophonic sound, which has an upper cut-off frequency less than the horizontal frequency, this buzz may not be of sufficient strength to be annoying.
Recently, interest has been shown in adding stereophonic sound to television receivers. It has been found with the intercarrier system that the amplitude modulation of the picture signal causes a buzz in the audio signal that is annoying due to the wider bandwidth of the stereophonic signal compared to that of the monophonic signal. To overcome this problem, an IF system called "quasi-parallel" or "quasi-split sound" has been proposed as shown in FIG. 7 of the article entitled "Intercarrier Buzz Phenomena Analysis and Cures" by P. Fockens and C. G. Eilers, IEEE Transactions on Consumer Electronics, Vol. CE-27, No. 3. In the quasi-parallel system, the video signal is applied to one IF channel for amplification and detection, while both the video and sound signals are applied to another channel in order to form the intercarrier signal. Filters are used at the input of the channels to select the desired signals. These filters have their inputs coupled together and to the output of the tuner mixer. Further, if the filters are of the SAW (surface acoustic wave) type, they may even share a common launch electrode. This may lead to interaction between the filters, thereby causing undesired peaks and nulls in the frequency response of the filters.
It is therefore desirable to have a television IF system without the above problems.